Saturday, June 20, 2009

National debt used to be the big number we all lived in fear of. Now it's greenhouse gases.

Climate change is likely to have all sorts of nasty consequences over the next century—among them, according to a brand-new report from the U.S. Global Change Research program, an increase in torrential downpours in the American northeast.

So it was uncomfortably fitting that a major climate-consciousness-raising event took place in just such a downpour. As reporters and dignitaries huddled under leaky tents just outside New York's Madison Square Garden on Thursday, Deutche Bank switched on its mammoth Carbon Counter billboard. The counter, towering 70 feet above busy Seventh Avenue and dramatically visible to hundreds of thousands of commuters who take the train to and from Penn Station, displays a real-time count of heat-trapping greenhouse gases we're pumping into the atmosphere—about 2 billion metric tons every month, added to the 3.6 trillion tons already floating around up there.

How do they know it's 2 billion tons? Actually, they know it isn't. Although carbon dioxide is by far the most significant human-generated greenhouse gas, it isn't the only one. Methane, generated by ruminating cows and rice paddies is another; nitrous oxide, created in making fertilizer, is another; so are halocarbons, used as refrigerants. If you really want to know about how much heat we're trapping, you have to take these into account too—and that's what Deutche Bank and its scientific advisers from MIT wanted to do.

It's complicated, though. For one thing, each of these gases traps heat at a different rate (OK, they really trap infrared radiation, but it ends up amounting to the same thing). Methane, for example, is a much more efficient energy-trapper than CO2; it's just that we emit a lot less of it. Each of these gases, moreover, degrades in the atmosphere at a different speed. That means you can't just add them up. "It's like you give someone a hundred dollars," says MIT atmospheric scientist Ron Prinn, "but it's a mix of Australian and Canadian and U.S. dollars. "You have to make some conversions before you know what it's worth." For the Carbon Counter, those conversions run into many pages of equations, at the end of which you get a number representing the "CO2 equivalent" of 20 different gases. Add them up, and you're at 2 billion tons monthly.

That's a big number, certainly, but what exactly does it mean? Most popular accounts of climate change don't talk about tons; they talk about parts-per-million—the number of CO2 or other molecules you'd find in a million molecules of atmosphere. CO2 was at about 280ppm back in 1700; it's now at 386 and rising. For perspective, climate scientists believe that if CO2 rises to 450ppm or so, the global average temperature could rise as much as 2 degrees Celsius, with serious consequences (and heavy rainstorms are hardly the worst).

But if you factor in the other greenhouse gases, we're already at 450, or pretty close to it. That being the case, you'd think we'd already be seeing dramatically rising seas and severe weather changes. There are two reasons why we aren't. First, it takes a while for heat to build up once the gases are up there. Second, and more important, the Carbon Counter doesn't take aerosols into account. These are tiny particles of soot, sulfur dioxide and other pollutants spewed into the air along with greenhouse gases. "The problem with these," says Bill Chameides, dean of Duke University's Nicholas School of the Environment, "is that some aerosols tend to cool the planet, some tend to warm it, and some interact with clouds in ways we don't understand."

That's the good news. The bad news is that aerosols cause their own problems— lung disease and acid rain, just to name a couple. Presumably, we'll be trying to limit those emissions in the future, which will leave the greenhouse gases to do their thing without interference.

By leaving some factors out, the Carbon Counter is by definition somewhat inaccurate. But since most of us don't know what 3.6 trillion tons of carbon or carbon-equivalent or whatever actually means, it hardly matters. It's a big number, and it's getting bigger, fast. Deutche Bank and the MIT folks hope that seeing these huge numbers scroll by on a giant billboard will make people more aware of what we're doing to the planet, just as billboards with the U.S. national debt try to raise awareness about another scary number.

Given how much people pay attention to the debt, though, let's hope this one is more effective.